In recent years, addition of various nanomaterials has been studied for purposes of downsizing and weight saving in various fields. In particular, for environmental or resource problems, carbon materials such as graphene, CNT (carbon nanotube) and fullerene have attracted attention as nonmetal nanomaterials, and a resin composite reinforcing material in which a reinforcing material (a filler) is dispersed in a resin for a purpose of improving physical properties of the resin (tensile strength, elastic modulus, etc.) has been proposed.
For example, a resin composite reinforcing material in which a carbon material such as flaked graphite is added to a thermoplastic resin such as polyolefin has been disclosed (Patent Literature 1). Further, a composite reinforcing material having flaked graphite and an inorganic filler added thereto for a purpose of improving physical properties (tensile elastic modulus, rigidity, and impact resistance) has been disclosed (Patent Literature 2 and Patent Literature 3).
Of these, graphene is superior to other carbon materials in aspect of mass productivity, handleability, etc., as well as performance, and expectations have been placed on graphene in various fields. However, when a reinforcing material such as graphene is kneaded into a resin, the reinforcing material needs to be dispersed uniformly in order to sufficiently exhibit an improvement effect of physical properties.
In order to obtain high-quality graphene which, for example, has fewer graphite layers, a method in which weak ultrasonic waves are applied to natural graphite in a solvent (NMP) for a long time (7-10 hours), large agglomerates which deposit on the bottom are then removed, and the supernatant is then centrifuged to concentrate it, thereby obtaining a graphene dispersion in which 20% or more of flakes are of a single layer, 40% or more of flakes are of double or triple layers, and less than 40% of flakes are of 10 layers or more of a graphite material and are dispersed at about 0.5 g/L, has been considered (Patent Literature 2).